Oxygen sensor

I have a 1997 Nissan hard body pickup with a 2.4 litter engine and standard 5 speed trans. It has aprox 95000 miles on it and in very good condition. The problem I'm having is the cat converter has been removed and the O2 sensor on the manifold keeps going out. I replaced it once and got 28 mpg for just a little while. My question is can I replace it a heavy duty one and will the absence of the cat make it go bad as well? I would rather not have to reinstall the cat as it is not a required item here.
By the way, I am now getting approx 20 mpg with the bad O2